Blood pressure is one of the key hemodynamic parameters that are measured. Direct measurement of blood pressure in blood vessels provides accuracy that is valuable in many instances, such as during surgical procedures. Direct measurement of blood pressure in blood vessels, however, is limited by current technology.
For surgical patients, a need can be present to measure pressure over an extended duration, exceeding several days. Extended measurements are limited by current technology, for they involve direct contact with the blood stream, where concerns arise with the risk of thrombus formation and septicemia. In addition to the risks presented to the patient, a direct blood contact sensor tends to become less accurate over time due to signal drift and the formation of a biolayer, which reduces sensitivity and defeats the purpose of direct in-vivo blood pressure measurements.
Millar Instruments makes in-vivo blood contact sensors (Millar Mikro-Tip® pressure catheters) that are widely used in clinical studies. The Millar Instruments' device includes a piezoresistive pressure sensor encased in a catheter tip, which is inserted into the blood vessel. Millar Instruments' PVR-1045 and SPR-1000 devices have ultra-miniature catheters that are intended for use in small animal research. The catheter tip size of ⅓ mm minimizes blood flow obstruction in the vessel being monitored. However, the catheter style device still raises a risk of thrombus formation and septicemia in human patients.
Deltran also makes a disposable pressure sensor for in-vivo use. U.S. Pat. No. 6,117,086 discloses a Deltran sensor, which includes a semiconductor strain gage connected to a fluid-filled catheter/manometer system. The catheter is coupled with a strain gage through a disposable dome having a compliant isolation media that contacts the strain gage. The dome is configured to provide an electrical and biological barrier between the fluid in communication with the invasive catheter and the strain gage's diaphragm. However, this approach does not minimize the complications stated above because the primary transduction method requires an invasive catheter. Other companies, e.g., Integra™, also make similar devices. The Camino® line of intracranial pressure sensors of Integra™ is another example of the catheter approach for in-vivo pressure measurement.
Improving pressure sensing monitoring remains an issue of interest in part due to the limitations of current commercially available devices. Examples of the research are in the following publications. Kalvesten, et al, “The First Surface Micromachined Pressure Sensor for Cardiovascular Pressure Measurements,” Micro Electro Mechanical Systems Proceedings, The Eleventh Annual International Workshop, p. 574-579, (Jan. 25-29 1998), discloses a device that uses a catheter, which risks thrombosis formation or septicemia. Chatzandroulis, S.; et al, “A Miniature Pressure System with a Capacitive Sensor and a Passive Telemetry Link for Use in Implantable Applications,” Journal of Micro Electro Mechanical Systems, vol. 9, no. 1 pp. 18-23, (March 2000) describes a device that uses a capacitive pressure sensor to be packaged within a catheter. The catheter approach, again, risks thrombosis formation or septicemia. Similar efforts are described in Hierold, C.; et al “Implantable Low Power Integrated Pressure Sensor System for Minimal Invasive Telemetric Patient Monitoring,” Micro Electro Mechanical Systems Proceedings, The Eleventh Annual International Workshop, pp. 574-579, (Jan. 25-29 1998), and in U.S. Pat. Nos. 4,718,426; 4,718,427; 4,718,428; 4,669,485; 6,471,656; 7,118,534; 7,048,691; and 7,017,420.